Method of producing a cast alloy coated oxidizable metal article



July 17, 1956 F CRAWFORD 2,754,570

METHOD OF PR CING A CAST ALLOY COATED OXIDIZABLE ME ARTICLE Filed April 1952 2 "mum,"

1571 5 TLfiZZIIT fieaeridt U Crawford United States Patent I METHOD OF PRODUCING A CAST ALLOY COATED OXIDIZABLE METAL ARTICLE Frederick C. Crawford, Cleveland, Ohio, assignor to Thompson Products, Inc., Cleveland, Ohio, :1 corporation of Ohio Application April 26, 1952, Serial No. 284,581

4 Claims. (Cl. 29156.8)

The present invention is concerned with the coating of oxidizable articles with an oxidation resistant coating, and is particularly directed to the coating of fluid directing members used in environments of high temperature and oxidative coating, such as parts for jet engines and the like.

The extensive development in the field of jet engines has necessitated the development of alloys for use in the manufacture of parts for such jet engines which can withstand the extremely high temperatures and oxidizing atmospheres normally present in the operation of turbo-jet engines. Such alloys, to function properly, must have a high strength, toughness, creep resistance and resistance to the oxidizing gases present in the turbine engine.

One relatively plentiful metal which exhibits good strength, toughness and creep resistance characteristics is molybdenum. However, the oxidation resistance of molybdenum and alloys containing major amounts of molybdenum is quite poor. Although metallic molybdenum has a melting point in excess of 4500 B, it begins oxidation at temperatures as low as 900 F., the rate of formation of the molybdenum oxide increasing with an increase in temperature. Since the molybdenum oxide sublimes, complete disintegration of the molybdenum body will'occur in a relatively short time under conditions of high temperature oxidation.

Many diiferent types of oxidation resistant coatings have been proposed for the protection of molybdenum surfaces which are to be exposed to high temperatures and oxidizing conditions. One of the difficulties encountered with such coatings has been that of securing a uniform thickness or predetermined thickness gradients along the surface of the molybdenum body, particularly where the molybdenum article is of a rather complex shape. The coating of a molybdenum turbine bucket with oxidation resistant coatings presents a'good example of the type of article upon which oxidation resistant coatings of predetermined thickness or a uniform thickness is difiicult, if not impossible to achieve.

The present invention provides a method for casting on oxidation resistant metals to a shaped molybdenum article. Basically, the process involves anchoring a preformed oxidizable article, such as a molybdenum turbine bucket in a pattern die and applying a relatively low melting pattern material about the article, using the article as a core in the pattern die. After solidification of the low melting pattern material as a coating about the exposed surfaces of the oxidizable core, the coated article is removed from the pattern die and invested in suitable ceramic mold-making compositions. After application of the mold-making compositions, and initial setting thereof, the low melting pattern material is removed, leaving the oxidizable article anchored in the ceramic mold-making composition with a molding cavity about the article being formed upon removal of the low meltingpattern material.

Subsequently, the mold-making material is set to rigidify the ceramic composition into a relatively hard, refractory; mold. [A molten oxidation-resistant metal or alloy is then introduced into the mold thus produced, under pressure,

to} fill the molding cavity thus resulting from removal of the pattern material.

Upon solidification of' the 'alloy' composition, the mold is broken away and the anchoring means removed to leave a cast-on coating of the oxidation resistant metal or alloy 0n the molybdenum surface of the desired thickness.

An object of the present invention is to provide an improved method for applying an oxidation resistant coating to an oxidizable metal.

Another object of the present invention is to provide an improved method for casting an oxidation resistant metal as a surface coating to an intricately shaped oxidizable metal base.

Another object of the present invention is to provide a method for manufacturing a ceramic mold, using a refractory, oxidizable metal to be coated as the core material in the manufacture of the mold.

Other objects and features of the present invention will be apparent to those skilled in the art from the following description of the attached sheet of drawings, which illustrate by way of preferred embodiment, the process of the present application as it is applied to the coating of a molybdenum turbine bucket.

In the drawings:

Figure 1 is a cross-sectional View of a preformed molybdenum turbine bucket provided with core prints for securing the same in a pattern die;

Figure 2 is a cross-sectional view of the turbine bucket supported within the pattern die;

Figure 3 is a cross-sectional view of the turbine bucket after application of the pattern material as a low melting coating thereon;

Figure 4 is a cross-sectional view of the coated turbine bucket after application of the ceramic mold-forming composition;

Figure 5 is a view similar to Figure 4 illustrating the maner in which the molybdenum turbine bucket is supported within the mold-making composition after removal of the low melting pattern material;

Figure 6 is a view similar to Figure 5 and illustrates the formation of the metallic coating shell about the turbine bucket within the ceramic mold; and

Figure 7 is a cross-sectional view of the final coated molybdenum turbine bucket.

In the drawings:

Refrence numeral 10 indicates generally a turbine bucket for jet engines, including a fir tree root portion 11 and an arcuately shaped vane portion 12. A seen in Figure 1, the root portion 11 is notched as indicated at numeral 13 to receive a core print 14 composed of an oxidation resistant material of the type to be hereinafter described. Similarly, the vane portion 12 is notched to receive a second core print 16 of similar oxidation resistant metal.

The turbine bucket 10 is supported by means of the core prints 14 and 16 in a split pattern die having complementary die sections 18 and 19. Each of the die sections 18 and 19 is suitably engraved on its inner surfaces to provide a molding cavity 20 conforming to the shape of the bucket 10. The die sections 18 and 19 are clamped together so as to tightly engage the core print 16 and thereby support the bucket 10 within the molding cavity 20 formed by the cooperating molding surfaces of the sections 18 and 19. The core print 16 is preferably tapered and received between the cooperating mold sections 18 and 19 so as to leave a thin void between the top of the vane 12 and the inner walls of the cooperating die sections cavity 20. "The core prin't14' extends beyond the. ate

22 and is tightly received between the cooperating mold sections 1s'and19: I I 3 After assembly of the metal sections 18 and 19 with the bucket supported therein by means of the core print 16, a molten relatively low melting pattern material is introduced into the molding cavity 20. In a preferred embodiment of the present invention, liquid mercury is employed as the pattern material, although other low melting pattern materials such as wax may also be used. Mercury, however, is preferred because it presents fewer problems of dimensional stability due to pattern shrinkage than does wax.

After the liquid mercury is introduced through the gate 22 into the molding cavity 20, the mold is subjected to a temperature sufficiently low to freeze the mercury into a solid coating completely enveloping the exposed surfaces of the turbine bucket I0. For best results, the temperature of the pattern should be considerably below the melting point of mercury, usually on the order of about minus 135 F. After freezing of the mercury in the pattern die, the die sections 18 and 19 are separated and the bucket 10, now having a coating 24 (Figure 3) of mercury thereon is removed from the molding cavity. As seen in Figure 3, the turbine bucket 10 thus has a continuous coating 24 on the exposed surfaces, the thickness of which depends upon the configuration of the molding cavity employed in the pattern die.

The mercury-coated bucket is then employed in the formation of a ceramic mold according to the process illustrated in Figures 4-7 of the drawings. The process of forming a mold-making ceramic composition about a frozen pattern involves successive applications of refractory particles, in suitable binders, to the surface of the frozen pattern material. In the first of these applications, finely divided refractory particles are applied in the form of slurry in a low temperature vehicle such. as a member of the Freon series, particularly (difiuoro monofluoro methane). In addition to the refractory particles, the initial coating of the frozen pattern also includes materials such as polyvinyl acetate and ethyl cellulose which are effective film-forming ingredients and function as low temperature binders in securing the applied coating to the surface of the frozen mercury. A typical analysis of the dry ingredients of the first coatings is the following:

Table 1 Percent by weight Polyvinyl acetate .75 Ethyl cellulose .25 Phenol-formaldehyde resin (A stage) 1.0 Sodium fluoride .75 Boric acid .25 Zirconite particles Balance Since the primary coating on the frozen pattern will ultimately define the finish of the casting surface, the refractory particles (zirconite) should be in a very fine state of subdivision, on the order of 325 mesh or less.

After mixture of the dry ingredients, a slurry is formed from the dry ingredients and a low temperature liquid vehicle of the type described, sufficient amounts of the dry materials being. added to yield a slurry having a specific gravity on the order of 2.3- to about 2.5.

The mercury coated pattern is dipped one or more times into a slurry of the type described, while maintai-ning the slurry temperature atv least as low as minus 55 C. After applications of one or more of the preliminary coatings, additional coatings can he applied of differing ceramic content and different composition. A typical intermediate dip composition follows:

Table II Percent by weight Polyvinyl. acetate 1.0 Ethyl cellulose .33 Phenolic resin .37 Ammonium dihydrogen phosphate 2.0 Zirconite' sand 36.42.

Zirconite flour Balance The above dry ingredients are added to the low temperature carrier to produce a very thick slurry. One or more dips of the coated pattern material into the above type of composition suffices to produce a good intermediate coating of the previously coated pattern material.

The final application of ceramic mold-making material should contain materials which are effective at high temperatures to rigidify and bond the refractory metal particles together into a self-sustaining, relatively rigid mold structure. Suitable high temperature binders include materials such as the phosphates, borates, fluorides, silicates, and the like. After one or more dips of compositions of the type indicated in the previous table, a final dip coat is applied to the outer surface of the coated pattern mate- The above identified composition is dispersed in the low temperature carrier as a very thick slurry into which a coated pattern material is dipped one or more times.

As a result of the foregoing dipping operations, a retractory mold-making composition is set up on the surface of the mercury pattern material, the compositions including sufficient amounts of low temperature binder material to keep the refractory particles coherently bound together even at the low temperatures employed, and containing. other ingredients which are effective upon firing of the refractory compositions to bond the refractory particles together at high temperatures.

After application of the coatings, through the various dips, the resulting refractory layer 26 (Figure 4) is on the order of one-sixteenth to one-quarter of an inch in thickness. The size of the refractory coating layer has been exaggerated in the drawings for purposes of clarity. As seen in Figure 4, the core print 16 is surrounded by and embedded in the refractory layer 26 and the mercury shell 24 similarly surrounded by the refractory mold-making composition.

After the refractory layer 26 has set initially, the assembly may be heated up to room temperature to permit melting of the mercury and draining off the mercury. The mercury leaves the surrounding refractory coating 26 through the gate 27 provided by the solidification of mer cury in the original gate 22 of the pattern die. Since the bucket It) is firmly held in the refractory coating 26 by means of the core print 16, the mercury is free to flow out of the composite structure, leaving a molding cavity 28 betweenthe surfaces of the bucket 10 and the inner walls of the refractory coating 26. After liberation of the mercury, the entire assembly is heated at a temperature sufii'ciently high to set the mold into its final rigid structure. Temperatures on the order of 1700" and 1900 F. are ordinarily employed for firing of the mold, with a temperature of 1850 F. being preferred. In order to protect the molybdenum bucket against oxidation, the firing should preferably be done in a non-oxidizing atmosphere such as hydrogen or in an atmosphere such as argon, helium, or the like.

The next step in the process consists in pouring amolten oxidation-resistant alloy which is to provide the oxidationresistant surface to the molybdenum bucket into the molding cavity provided by elimination of the mercury pattern materiaL Many different types of oxidation-resistant alloys can be cast into the molding cavity to provide the type of coating desired. An excellent oxidation-resistant alloy for the purposes of this invention is one of the type described. in Turner and Ruppender, U. S. application Serial No. 214,116, entitled Corrosion and Impact Resistant Article and Method of Making Same, assigned to the same assignee as the present application. The alloy therein disclosed has a composition within the following range:

Table IV Other suitable alloys include corrosion resistant alloys such as AMS-5373A (a cobalt-chromium-tungsten-carbon-silicon alloy), and inconel alloys (containing about 77.75% nickel, 13.5% chromium, 6.0% iron, and 2.0% silicon) or other specialized castable alloys having good oxidation and corrosion resistance.

Since the cavity 28 will usually be quite thin, being on the order of A and since the molten alloy must follow a rather tortuous path in filling up the cavity, it is preferable that the molten alloy be introduced into the molding cavity under pressure, using either centrifugal or pressure casting procedures.

After the molding cavity 28 has been filled with the molten metal, to provide an outer coating 30 about the exposed surfaces of the molybdenum bucket, and also about portions of the core prints 14 and 16, the mold is cooled to solidify the metal within the cavity. As the final step, the ceramic mold is broken away and excess core print material from core prints 14 and 16 is removed to produce a turbine bucket of the type illustrated in Figure 7 in which the molybdenum turbine bucket body 10 is provided with a continuous metallic protective shell 30 while retaining the structural strength of the underlying molybdenum metal. In a preferred embodiment of the invention, the oxidation-resistant alloy cast about the molybdenum turbine bucket is the same employed initially for the core prints 14 and 16 so that the molten alloy when poured into the cavity readily bonds to the core rints.

p While the foregoing description has been made primarily in conjunction with a process using mercury as a pattern material, the process is equally applicable to the use of the more conventional pattern materials such as wax, although molds produced from wax patterns are not as dimensionally accurate as those produced from a mercury pattern. In the case of wax patterns, the moldmaking composition ordinarily includes finely divided silica sand and a binder such as an organic silicate which binds the particles together.

From the foregoing, it will be appreciated that the present invention provides an improved process for applying by casting an oxidation resistant coating to an oxidizable metal base of intricate shape. One of the main advantages of the process over ordinary coat ng procedures is the fact that the dimensions of the applied coating can be carefully controlled to within close tolerances, even where the article being coated has undercuts and other discontinuities which render uniform coating of metallic objects difiicult.

It will be appreciated that various modifications may be effected without departing from the scope of the novel concepts of the present invention.

I claim as my invention:

1. The method of coating an oxidizable fluid directing member which comprises attaching an oxidation-resistant metal anchoring means to said member, positioning said member in a pattern die contacting only the anchoring means and not the die and with said member held in position within said die by said anchoring means solely,

said die having walls therein defining a molding cavity entirely surrounding said fluid directing, member, introducing a fluid relatively low melting pattern material into said molding cavity, freezing said pattern material about said member to produce a coating of said pattern material about said member, removing the resulting coated member from said die, applying a heat-hardenable refractory mold-making composition about said coated member, and about said anchoring means, melting out said pattern material, heating said mold-making composition to a temperature sufiicient to set said composition into a relatively rigid mold structure in which said anchoring means is anchored in said mold structure, casting a molten oxidation-resistant metallic composition into the space between said member and the internal walls of said mold structure to secure a portion of the anchoring means in engagement with said member, and trimming off the remaining portion of the anchoring means extending outwardly from the cast composition on said member.

2. The method of coating an oxidizable fluid directing member which comprises attaching an oxidation-resistant metal anchoring means to said member, positioning said member in a pattern die with said member held in position within said die but free from contact therewith by said anchoring means, said die having walls therein defining a molding cavity about said fluid directing member, introducing liquid mercury into said molding cavity, freezing said mercury about said member to produce a coating of mercury about said member, removing the resulting mercury coated member from said die, applying a heathardenable refractory mold-making composition about said coated member and about said anchoring means, melting out said mercury, heating said mold-making composition to a temperature sufiicient to set said composition into a relatively rigid mold structure in which said anchoring means is anchored in said mold structure, casting a molten oxidation-resistant metallic composition into the space between said member and the internal walls of said mold structure, to secure a portion of the anchoring means in engagement with said member, and trimming off the remaining portion of the anchoring means extending outwardly from the cast composition of said member.

3. The method of coating a molybdenum turbine bucket which comprises attaching a core print of oxidationresistant metal at one end of said bucket and another at the opposite end of said bucket, positioning said bucket in a pattern die with said core prints alone engaging said die to support said bucket therein free from contact with said die, said die having walls therein defining a molding cavity about said bucket, introducing a fluid relatively low melting pattern material into said molding cavity, freezing said pattern about said bucket to produce a coating of said pattern material about said bucket, removing the resulting coated bucket from said die, applying a heat-hardenable mold-making composition about said coated member, and about said core print, melting out said pattern material, heating said moldmaking composition in a reducing atmosphere to a temperature sufficient to set said composition into a relatively rigid mold structure in which said core print is anchored in said mold structure, casting a molten oxidation-resistant alloy composition into the space between said member and the internal Walls of said mold structure, removing the mold structure, and trimming oif the ends of said core prints extending outwardly from the alloycoated member.

4. The method of coating a molybdenum turbine bucket which comprises attaching a core print of oxidation-resistant metal in one end of said bucket, positioning said bucket in a pattern die with said core print engaging said die to support said bucket therein free from contact with said die, said die having walls therein defining a molding cavity about said bucket, introducing liquid mercury into said molding cavity, freezing said mercury about said bucket to produce a coating of mercury about said bucket, removing the resulting coated bucket from said'die, applying a heat-hardenable' mold-making composition about said coated member, and about said core print, melting out said mercury, heating said mold-making composition in a reducing atmosphere to a temperature sufficient to set said composition into a relatively rigid mold: structure in which said core print is anchored in said mold structure, casting a molten oxidation-resistant alloy composition under pressure into the space between said member and the internal Walls of said mold structure to secure the core print in engagement with said member, and trimming off the portion of the core print extending outwardly from the alloy-coating on said member.

1,673,973 Drevitson June 19, 1928 8 Harris et a1. Sept. 12, Schupp Nov. 7, Fahlrnan June 12,. Thompson Dec. 20, Derby Jan. 24, Eccles June 18, Zahn Nov. 14, Kohl May 21, Zahn et a1. May 20, Brennan Nov. 21,

FOREIGN PATENTS France Feb. 3, France Oct. 29, 

1. THE METHOD OF COATING AN OXIDIZABLE FLUID DIRECTING MEMBER WHICH COMPRISES ATTACHING AN OXIDATION-RESISTANT METAL ANCHORING MEANS TO SAID MEMBER, POSITIONING SAID MEMBER IN A PATTERN DIE CONTACTING ONLY THE ANCHORING MEANS AND NOT THE DIE AND WITH SAID MEMBER HELD IN POSITION WITHIN SAID DIE BY SAID ANCHORING MEANS SOLELY SAID DIE HAVING WALLS THEREIN DEFINING A MOLDING CAVITY ENTIRELY SURROUNDING SAID FLUID DIRECTING MEMBER, INTRODUCING A FLUID RELATIVELY LOW MELTING PATTERN MATERIAL ABOUT SAID MOLDING CAVITY, FREEZING SAID PATTERN MATERIAL ABOUT SAID MEMBER TO PRODUCE A COATING OF SAID PATTERN MATERIAL ABOUT SAID MEMBER, REMOVING THE RESULTING COATED MEMBER FROM SAID DIE, APPLYING A HEAT-HARDENABLE REFRACTORY MOLD-MAKING COMPOSITION ABOUT SAID COATED MEMBER, AND ABOUT SAID ANCHORING MEANS, MELTING OUT SAID PATTERN MATERIAL, HEATING SAID MOLD-MAKING COMPOSITION TO A TEMPERATURE SUFFICIENT TO SET SAID COMPOSITION INTO A RELATIVELY RIGID MOLD STRUCTURE IN WHICH SAID ANCHORING MEANS IS ANCHORED IN SAID MOLD STRUCTURE, CASTING A MOLTEN OXIDATION-RESISTANT METALLIC COMPOSITION INTO A SPACE BETWEEN SAID MEMBER AND THE INTERNAL WALLS OF SAID MOLD STRUCTURE TO SECURE A PORTION OF THE ANCHORING MEANS IN ENGAGEMENT WITH SAID MEMBER, AND TRIMMING OFF THE REMAINING PORTION OF THE ANCHORING MEANS EXTENDING OUTWARDLY FROM THE CASE COMPOSITION ON SAID MEMBER. 